The present invention relates to a method for determining the braking-surface temperature for one or more wheels of a motor vehicle by continuously redetermining the braking-surface temperature (T) cyclically in the driving mode indirectly from wheel-speed and brake-state data, in each cycle, reducing the instantaneous temperature value (T) by a cooling decrement (dT.sub.k) to no more than a predetermined minimum temperature (T.sub.min), and, with the wheel brake activated during normal braking or braking controlling wheel slip, increasing the instantaneous temperature value (T) by a heating increment (dT.sub.h) ascertained in dependence on data from at least one of measured wheel speeds (Rd), from quantities (v, a) derivable therefrom and from slip-controlling wheel-brake valve activations. The term "braking surface" as used herein means both a brake lining and a brake disc.
Particularly in modern systems of driving dynamics which, in order to obtain driving stability or to increase traction, make use of a drive-slip control system, by way of which brake pressure can actively be built up in one or more driving wheels, information must be available on the braking-surface temperature. Such information is necessary in the case of a high wheel-brake load as a result of long-lasting control actions involving the system of driving dynamics, for example on account of an unfavorable driving mode, to protect the wheel brakes against overloading and to ensure that the normal braking function is maintained.
DE 31 27 302 C2 describes measurement of the braking-surface temperature directly by the use of appropriate sensors. To avoid the need for temperature sensors, Preliminary publication DE 34 07 716 A1 describes deriving the braking-surface temperature for a braking operation, for the purpose of ascertaining the brake wear, from the actuation time of the brake, the wheel speed or the vehicle speed and the brake pressure recorded by a specific sensor.
A further indirect temperature-determining method is described in the German Patent Application P 43 16 993.7 not previously published. In this method, the braking-surface temperature is obtained from a consideration of the energy balance by relevant data ascertained during a braking operation. For the cooling interval following each braking operation, a cooling behavior exponential in time is assumed.
DE 42 35 364 A1 also discloses a method for the indirect determination of the temperature of a disc brake. The temperature value is re-determined cyclically, and is increased during braking operations and reduced during time intervals without braking. The temperature increase during braking is ascertained here by a load signal which represents the load state of the brake and for the determination of which are employed once again, inter alia, a speedometer signal representing the vehicle speed and a brake-pressure signal representing the brake-jaw pressing force. This brake-pressure signal is derived from a pressure sensor to be arranged, for example, on a brake valve.
A known method without brake-pressure sensors Is described in DE-4,020,693 A1. The temperature increase associated with a normal braking operation not controlling wheel-slip is ascertained in dependence on the vehicle speed, on the vehicle deceleration and on vehicle-specific constants. The vehicle speed and, derived therefrom, the vehicle deceleration are ascertained via wheel-speed sensors. In particular, the temperature-increase increment consists of two addends, of which one is proportional to the square of the permanently predetermined low reference speed and the other to the speed change since the last determination of the increase increment. Thus, no speed-dependent road-resistance effect is taken into account, thereby limiting the accuracy of the brake-temperature determination.
An object of the present invention is to provide a method in which the braking-surface temperature can be comparatively reliably estimated continuously without temperature and brake-pressure sensors, specifically also for a driving wheel loadable with brake pressure by a wheel-slip control system.
This object has been achieved by a method in which the heating increment (dT.sub.hn) for normal braking operations is fixed proportionally to a braking-induced vehicle-deceleration value (a) obtained as the difference between deceleration value ascertained from the wheel speeds and a deceleration value taken from a stored road-resistance characteristic diagram taking into account at least one of engine drag moment, the air resistance and rolling resistance of the vehicle, and proportionally to measured speed (Rd) of the wheel.
The only sensor required for practicing the method of the present invention is essentially that for recording wheel speeds which is in any case usually present in currently produced vehicles. The method involves cyclic temperature re-determinations. The brake cooling is taken into account from cycle to cycle by subtraction of a respective cooling decrement. This arithmetic decrementing is discontinued when the temperature thus ascertained falls below a predetermined minimum value which is expediently of the order of magnitude of the ambient temperature, so that no unrealistically low temperature values are obtained.
In the temperature-determining cycles occurring within brake-activation phases, a heating increment is to be added to the instantaneous temperature value and is derived from the vehicle deceleration obtained via the recorded wheel speeds and/or from control information for the brake valves of the wheel for braking operations in response to wheel-slip control operations. In this way, the braking-surface temperature of one or more wheels, particularly also of driving wheels loadable actively with brake pressure via a wheel-slip control system, can be reliably estimated at any moment at a minimum outlay in terms of sensors, data in any case present on the vehicle being for the most part utilized.
For the normal braking phases, provision is made for correcting the ascertained vehicle-deceleration value by a deceleration value which is governed by the road resistance, in particular by the engine drag moment, but also by the air resistance and rolling resistance of the vehicle. This deceleration value induced by road resistance can, for example, be determined empirically and be stored retrievably in the vehicle in the form of corresponding characteristic diagrams. The heating increment is then selected, for the normal braking operations, proportionally to the corrected vehicle-deceleration value and proportionally to the speed of the respective wheel.
According to a further aspect of the present invention, the cooling increment is selected not only in dependence on temperature, but also in dependence on vehicle speed. This makes possible an allowance for the fact that the cooling rate rises with an increase in vehicle speed and therefore also an increase in wheel speed.
For a wheel which is loadable actively with brake pressure by a wheel-slip control system, an advantageous feature of the present invention makes use, for slip-controlling braking operations, of the control information for the brake valves involved which is provided for this purpose. Thereby, it is possible to determine in a highly reliable way the built-up brake pressure which is itself a measure of the increase in the braking-surface temperature. In this case, during braking phases controlling wheel slip, the heating increment is selected proportionally to the brake pressure ascertained from the valve-activation data.
It is also advantageous to select the proportionality constant for one or both types of heating increments in dependence on temperature, specifically falling with a decreasing temperature, in order to allow for the fact that, at higher braking-surface temperatures, the temperature rise may no longer take place linearly with the temperature.
The present invention makes particular advantageous use of the temperature-determining method for controlling the activatability of an existing wheel-slip control system. For this purpose, when a corresponding limit value is exceeded by the braking-surface temperature, blocking information is generated. In such a highly heated brake, the method prevents the wheel-slip control from additionally loading the highly heated brake with brake pressure. This avoids excessive overheating of the braking surface, so that the braking surface can always still be kept operational for normal braking. Only when the braking-surface temperature has fallen again below a second limit value which is no higher than the first is the wheel-slip control cleared again. So as not to have to discontinue a renewed attempt at control after a short time because the threshold is exceeded, but instead to guarantee a sufficiently long cooling phase until the blocking information is cancelled again, or in order to keep effects of control oscillations low, the second limit value is preferably selected lower by a particular amount than the first limit value.
As a result of the foregoing, the preservation of the information on the braking-surface temperature is maintained even when the vehicle is parked, until the estimated temperature has reached a normal environment-related value which realistically corresponds to the actual brake-surface temperature after a sufficient period of standstill of the vehicle. After the vehicle has been at a standstill and is started up again shortly thereafter, the assumption of a value for the braking-surface temperature which corresponds essentially to the ambient temperature is prevented, where the braking surface has, in reality, not yet cooled to this temperature again.